The present invention relates to a lens system of an inner focus type that can be focused at short range. Furthermore, the present invention relates to an image pickup device equipped with a lens system of an inner focus type that can be focused at short range.
Conventionally, a lens system of an inner focus type capable of being focused at short range by disposing a plurality of focus-lens groups inside the lens system has been known in, for example, JP-A-3-278012, JP-A-4-110811 and JP-A-2003-329924. Such a configuration realizes a lens system that makes it difficult for dust to get into the lens system and for a lens to collide with an object even during a focusing operation.
Meanwhile, in recent years, instead of a conventional focus detection method of a phase difference method, a focusing type has been increasingly used; according to the focusing type, a change in contrast at a time when a lens group is transferred is read, and a focus state comes when the contrast approaches a maximum. Such a focusing type makes it unnecessary to dispose a reflection surface between a lens system and an image pickup plane. Therefore, in a constant volume inside the lens system when used. Therefore, the above configuration is favorable to keeping dust or water from getting into and reducing the noise that occurs as the lens groups move.
Moreover, a plurality of lens groups move to carryout inner focusing. Therefore, as the refractive power of the focusing lens groups increases, the above configuration becomes favorable to reducing the traveling distance of the focusing lens groups shifting from focused-at-infinity mode to focused-at-close-range mode.
Meanwhile, as the refractive power of the lens groups increases, a change in aberration and the like are more likely to occur when the lens groups move. In order to suppress the above, the first focusing lens group and the second focusing lens group are each made up of a plurality of lenses.
Moreover, the lens group that performs a wobbling operation is provided separately from the first focusing lens group and the second focusing lens group, and is formed so as to have a small thickness on the optical axis.
The above configuration makes lighter the lens group that performs the wobbling operation, and is favorable to increasing the speed of focusing control and saving energy.
It is preferred that the above invention satisfy one or a plurality of the following configurations at the same time.
Moreover, it is preferred that the following conditional expression (1) be satisfied:0.03<Dwob/(DF1+DF2)<0.15  (1)where
Dwob is the thickness on the optical axis of the wobbling the focusing type is advantageous to making the entire device smaller when the lens system is incorporated.
Moreover, in recent years, the following lens system has been also known: the lens system with a higher auto-focusing speed, which is so formed as to cause some lenses of the lens system to slightly vibrate, or wobble, in an optical-axis direction and determine a movement direction of a focusing lens group on the basis of a change in contrast on an image pickup plane.